1. Field of the Invention
This invention relates to new precipitated silica for thickening and providing thixotropy in liquid systems and to a process for their manufacture. The new precipitated silica shows superior efficiency at thickening and creating thixotropy compared to existing grades of precipitated silica. The new precipitated silica functions well as a thixotrope in non-polar or intermediate polar liquid systems such as unsaturated polyester (UPE) resin systems.
2. Discussion of the Background
Hydrophilic fumed and precipitated silica is well known to thicken and create thixotropy in liquid systems. The rheological effects are based on the formation of hydrogen bonding between silanol groups on adjacent silica particles. Hydrophilic silica has the greatest thickening and thixotropic effect in non-polar liquids, i.e., liquids that have a low amount of inherent hydrogen bonding. Inherent hydrogen bonding of a liquid medium can interfere with the hydrogen bonding between silanol groups of the silica and reduce the thickening effect. In intermediate polarity systems such as some epoxies, the thickening effect is still quite strong. However, in highly polar systems such as low molecular weight (mw) alcohols, hydrophilic silica is very ineffective at thickening and creating thixotropy. Examples of liquid systems where hydrophilic silica can be best utilized include aliphatic hydrocarbons, aromatic hydrocarbons, chlorinated solvents, fluorocarbons, mineral oils, terpenes, oil modified alkyds, epoxies, high mw esters, high mw ethers, glycol ether esters, unsaturated polyesters, silicones, vegetable oils, and vinyls. In some cases, hydrophilic silica could also possibly be used in even higher polarity systems such as high mw alcohols, high mw aldehydes, alkyds, high mw amides, high mw amines, glycol ethers, polyurethanes, vinyl esters, and thiols.
Unsaturated polyester resin systems are one the most common and important liquid resin systems thickened by hydrophilic silica. Unsaturated polyester compositions which are condensation polymers based on glycols and dibasic acids are used in a multitude of applications. These unsaturated polyester resins are essentially Newtonian in nature, that is, their viscosity is not shear dependent. In many of the applications of such resins, the resin must not drain from vertical surfaces before cure and must possess other properties of acceptable thixotropic character, which means that the resin should exhibit high apparent viscosity at low shear and much lower viscosity at high shear, and must also not drain from a vertical surface before cure.
Thixotropic polyester resins and gel coats are widely used in the marine industry as fiberglass reinforced plastics for boat building. Other typical applications for thixotropic resins include corrosion-resistant products, electrical components, tub and shower, wind mills and transportation. See “Modern Plastics Encyclopedia”, 1977-1978, p. 65-66; and 1978-1979, p. 56-59. These are special fields in which thixotropic resins must be used which meet the requirements set forth above for thixotropic character (see Connolly, “Modern Plastics Encyclopedia”, 1975-1976, p. 61-62).
The use of fumed silica in thixotropic unsaturated polyester (UPE) resins, is an old and very well known technology. Precipitated silica has also been utilized from at least the early 1980's. In 1985, the J. M. Huber Corporation was granted a U.S. Pat. No. 4,497,918 for utilizing combinations of precipitated and fumed silica in thixotropic unsaturated polyester formulations. Similar marine gel coats are disclosed in US 200710001343.
Compared to typical precipitated silica, fumed silica is more efficient at adding viscosity, is more thixotropic, provides better suspension stability in low viscosity resins and leads to better clarity. On the other hand advantages of precipitated silica compared to fumed silica include faster and shear independent dispersion, lower cost, better leveling of the coating or casting, less porosity in gel coats and in some cases less viscosity drift. As consequence in many cases, a blend of fumed and precipitated silica is utilized to achieve the benefits of both silica types. This, however, is often not an acceptable situation for manufacturers of unsaturated polyesters because two different kinds of silica have to be purchased and stored, which causes the need of two different storing devices. In addition, it is still necessary to use at least partly fumed silica having a much higher price compared to precipitated silica.
In summary, therefore, it can be stated that there is a strong need for new fillers for liquid systems, preferably unpolar and intermediate-polar systems, particular preferably UPE formulations, suitable to reduce costs, improve application properties and to simplify the production process.